7.2 volumes applications of integration in this section, we will learn about: using integration to...
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7.2
Volumes
APPLICATIONS OF INTEGRATION
In this section, we will learn about:
Using integration to find out
the volume of a solid.
In trying to find the volume of a solid,
we face the same type of problem as
in finding areas.We have an intuitive idea of what volume
means.
However, we must make this idea precise
by using calculus to give an exact definition
of volume.
VOLUMES
We start with a simple type of solid
called a cylinder or, more precisely,
a right cylinder.
VOLUMES
In general, if the area of the base is A and the height of the cylinder (the distance from B1 to B2) is h, then the volume V of the cylinder is defined as:
V = Ah
CYLINDERS
In particular, if the base is a circle with
radius r, then the cylinder is a circular
cylinder with volume V = πr2h.
CYLINDERS
For a solid S that isn’t a cylinder, we first
‘cut’ S into pieces and approximate each
piece by a cylinder.
We estimate the volume of S by adding the volumes of the cylinders.
We arrive at the exact volume of S through a limiting process in which the number of pieces becomes large.
IRREGULAR SOLIDS
For a solid S that isn’t a cylinder, we first ‘cut’ S into pieces and approximate each piece by a cylinder. We estimate the volume of S by adding the volumes
of the cylinders. We arrive at the exact volume of S through a limiting process in which the
number of pieces becomes large.
We start by intersecting S with a plane and obtaining a plane region that is called a cross-section of S.
IRREGULAR SOLIDS
Let A(x) be the area of the cross-section of S
in a plane Px perpendicular to the x-axis and
passing through the point x, where a ≤ x ≤ b.
Think of slicing S with a knife through x and computing the area of this slice.
IRREGULAR SOLIDS
We divide S into n ‘slabs’ of equal width ∆x
using the planes Px1, Px2, . . . to slice the solid.
Think of slicing a loaf of bread.
IRREGULAR SOLIDS
If we choose sample points xi* in [xi - 1, xi], we
can approximate the i th slab Si (the part of S
that lies between the planes and ) by a
cylinder with base area A(xi*) and ‘height’ ∆x.
1ixP ix
P
IRREGULAR SOLIDS
So, an approximation to our intuitive
conception of the volume of the i th slab Si
is:
( ) ( *) i iV S A x x
IRREGULAR SOLIDS
Adding the volumes of these slabs, we get an approximation to the total volume (that is, what we think of intuitively as the volume):
This approximation appears to become better and better as n → ∞.
Think of the slices as becoming thinner and thinner.Therefore, we define the volume as the limit of these sums as n → ∞).
We recognize the limit of Riemann sums as a definite integral and so we have the following definition.
1
( *)n
ii
V A x x
IRREGULAR SOLIDS
Let S be a solid that lies between x = a
and x = b.
If the cross-sectional area of S in the plane Px,
through x and perpendicular to the x-axis,
is A(x), where A is a continuous function, then
the volume of S is:
1
lim ( *) ( )n b
i axi
V A x x A x dx
DEFINITION OF VOLUME. DISK METHOD.
Notice that, for a cylinder, the cross-sectional
area is constant: A(x) = A for all x.
So, our definition of volume gives:
This agrees with the formula V = Ah.
b
aV Adx A b a
VOLUMES
Show that the volume of a sphere
of radius r is 34
3 .V r
Example 1SPHERES
If we place the sphere so that its center is
at the origin, then the plane Px intersects
the sphere in a circle whose radius, from the
Pythagorean Theorem,
is:2 2y r x
Example 1SPHERES
So, the cross-sectional area is:2 2 2( ) ( )A x y r x
Example 1SPHERES
Using the definition of volume with a = -r and
b = r, we have:
(The integrand is
even.)
2 2
2 2
0
3 32 3
0
343
( )
2 ( )
2 23 3
r r
r r
r
r
V A x dx r x dx
r x dx
x rr x r
r
Example 1SPHERES
The figure illustrates the definition of volume when the solid is a sphere with radius r = 1.
Notice that as we increase the number of approximating cylinders, the corresponding Riemann sums become
closer to the true volume.
SPHERES
Find the volume of the solid obtained by
rotating about the x-axis the region under
the curve from 0 to 1.
Illustrate the definition of volume by sketching
a typical approximating cylinder.
y x
Example 2VOLUMES
The region is shown in the first figure.
If we rotate about the x-axis, we get the solid
shown in the next figure. When we slice through the point x, we get a disk
with radius .
VOLUMES
x
Example 2
The area of the cross-section is:
The volume of the approximating cylinder
(a disk with thickness ∆x) is:
2( ) ( )A x x x
( )A x x x x
Example 2VOLUMES
The solid lies between x = 0 and x = 1.
So, its volume is:1
0
1
0
12
0
( )
2 2
V A x dx
xdx
x
VOLUMES Example 2
Find the volume of the solid obtained
by rotating the region bounded by y = x3,
y = 8, and x = 0 about the y-axis.
Example 3VOLUMES
As the region is rotated about the y-axis, it
makes sense to slice the solid perpendicular
to the y-axis and thus to integrate with
respect to y.
Slicing at height y, we get a circular disk with radius x, where
VOLUMES Example 3
3x y
So, the area of a cross-section through y is:
The volume of the approximating
cylinder is:
2 2 2/33( ) ( )A y x y y
2/3( )A y y y y
Example 3VOLUMES
Since the solid lies between y = 0 and
y = 8, its volume is:
8
0
8 2 3
0
853 35
0
( )
96
5
V A y dy
y dy
y
Example 3VOLUMES
The region R enclosed by the curves y = x
and y = x2 is rotated about the x-axis.
Find the volume of the resulting solid.
Example 4VOLUMES – WASHERS METHOD
The curves y = x and y = x2 intersect at
the points (0, 0) and (1, 1).
The region between them, the solid of rotation, and cross-section perpendicular to the x-axis are shown.
VOLUMES Example 4
A cross-section in the plane Px has the shape
of a washer (an annular ring) with inner
radius x2 and outer radius x.
Example 4VOLUMES
Thus, we find the cross-sectional area by
subtracting the area of the inner circle from
the area of the outer circle:
2 2 2
2 4
( ) ( )
( )
A x x x
x x
VOLUMES Example 4
Thus, we have:1
0
1 2 4
0
13 5
0
( )
( )
3 5
2
15
V A x dx
x x dx
x x
Example 4VOLUMES
Find the volume of the solid obtained
by rotating the region in Example 4
about the line y = 2.
Example 5VOLUMES
Again, the cross-section is a washer.
This time, though, the inner radius is 2 – x
and the outer radius is 2 – x2.
VOLUMES Example 5
The cross-sectional area is:
2 2 2( ) (2 ) (2 )A x x x
Example 5VOLUMES
So, the volume is:
1
0
1 22 2
0
1 4 2
0
15 3 2
0
( )
2 (2 )
5 4
85 4
5 3 2 5
V A x dx
x x dx
x x x dx
x x x
VOLUMES Example 5
The solids in Examples 1–5 are all
called solids of revolution because
they are obtained by revolving a region
about a line.
SOLIDS OF REVOLUTION
In general, we calculate the volume of
a solid of revolution by using the basic
defining formula
( ) orb d
a cV A x dx V A y dy
SOLIDS OF REVOLUTION
We find the cross-sectional area
A(x) or A(y) in one of the following
two ways.
SOLIDS OF REVOLUTION
If the cross-section is a disk, we find
the radius of the disk (in terms of x or y)
and use:
A = π(radius)2
WAY 1
If the cross-section is a washer, we first find
the inner radius rin and outer radius rout from
a sketch. Then, we subtract the area of the inner disk from
the area of the outer disk to obtain: A = π(outer radius)2 – π(outer radius)2
WAY 2
Find the volume of the solid obtained
by rotating the region in Example 4
about the line x = -1.
Example 6SOLIDS OF REVOLUTION
The figure shows the horizontal cross-section.
It is a washer with inner radius 1 + y and
outer radius
Example 6
1 .y
SOLIDS OF REVOLUTION
So, the cross-sectional area is:
2 2
2 2
( ) (outer radius) (inner radius)
1 1
A y
y y
Example 6SOLIDS OF REVOLUTION
The volume is:
1
0
21 2
0
1 2
0
13 2 32
0
( )
1 1
2
4
3 2 3 2
V A y dy
y y dy
y y y dy
y y y
Example 6SOLIDS OF REVOLUTION